The present invention relates to heat pumps and more particularly to radiant heat pumps utilizing radiative semiconductors.
A radiant heat engine can be defined as a Carnot heat engine in which one heat sink is in thermal contact with a radiating surface obeying Kirchhoff's law and the other heat sink is a general environment at room temperature (i.e. in the order of 300 K.). Work can be performed by such a radiant heat engine if the incident radiant energy received by the radiating surface differs from the black body radiation of the general environment. Such an engine can also absorb work and produce radiant heat fluxes different from that of a black body at the environmental temperature. In this mode the device is a radiative heat pump.
More particularly, the radiative heat pump system contemplated by the present invention is an enclosed chamber having a wall which is thermally conductive and in thermal contact with an external heat sink and in which the other heat sink is the contents of the chamber. A radiative semiconductor, similar in principle to a light emitting diode (LED), but operating in the thermal infrared region (5 to 40 microns wavelength), is placed in thermal contact with the thermally conductive chamber wall and positioned to radiate energy into the interior of the chamber.
If the external heat sink and the contents of the chamber are at the same temperature, and the semiconductor is not operating, the system will be in equilibrium with the black body radiation from the interior of the chamber which is absorbed by the semiconductor being balanced by the thermal radiation emitted from the semiconductor.
If the semiconductor is now operated to cause its infrared radiation to increase and exceed the black body radiation from the chamber, then the chamber contents will absorb such excess radiation and increase in temperature. It is known that LED's can be configured so that more energy in the form of light is emitted from them than is consumed in the form of electricity. The additional energy is provided by the semiconductor lattice in the form of heat, and heat removal by radiation results in lowering of the lattice's temperature. Such cooling of the semiconductor will cause it to absorb heat from the external heat sink so that part of the heat pumped into the chamber comes from the external heat sink.
A radiative heat pump should preferably be reversible in operation so that it could be used to cool the contents of the chamber. Thus, if the semiconductor is operated so that it will radiate less energy in the thermal infrared region than that emitted by the chamber, then the semiconductor will absorb more energy from the chamber interior than will be radiated from the semiconductor back into the chamber. The net loss of energy from the chamber interior will cause its temperature to decrease. Conversely, the net absorption of energy by the semiconductor will cause its temperature to increase, and it will give up heat to the external heat sink. If the electrical energy required for operation is less than the amount of energy given up by the semiconductor to the exterior heat sink, then the system operates to pump heat from the chamber interior to the exterior heat sink.
As mentioned above, it is known that LED's can be operated so that more energy in the form of light is emitted from them than is consumed in the form of electrical energy. For example, see U.S. Pat. No. 2,898,743 to Bradley. As such, refrigeration produced by such LED's is theoretically possible, although to date a net cooling effect has not been observed with such devices. Insofar as is known, there has been no discussion in the prior art of the operation of a semiconductor in such manner that it will produce less radiant emission than normal so that it can be a net absorber of black body radiation and produce cooling by such absorption.
Laser-pumped fluorescence can also achieve a cooling effect by an excess of radiant energy, but again, a net cooling effect has not yet been observed with this approach, and the operation is not reversible.
In principle, the LED and laser-pumped fluorescence approaches can pump heat, with the maximum coefficient of performance (COP) being limited only by the second law of thermodynamics. That is, the Carnot COP, (T.sub.1 /T.sub.2 -T.sub.1), T.sub.1 and T.sub.2 being the heat sink temperatures, is the limit of performance. The net rate of heat transfer is proportional to a statistical Boltzmann factor F, F=exp (-E.sub.gap /kT), where E.sub.gap is the semiconductoring gap energy for the LED, and kT is about 1/40 eV at 300 K. For typical LED semiconductors with E.sub.gap in the order of 1 eV, the factor F is about 4.times.10.sup.-18. Thus good thermodynamic performance (i.e. a high COP) is incompatible with a high rate of heat pumping.